Mon Dec 18 13:40:37 1995 Roland McGrath <roland@churchy.gnu.ai.mit.edu>

[glibc.git] / manual / time.texi

@node Date and Time, Non-Local Exits, Arithmetic, Top
@chapter Date and Time

This chapter describes functions for manipulating dates and times,
including functions for determining what the current time is and
conversion between different time representations.

The time functions fall into three main categories:

@itemize @bullet
@item 
Functions for measuring elapsed CPU time are discussed in @ref{Processor
Time}.

@item
Functions for measuring absolute clock or calendar time are discussed in
@ref{Calendar Time}.

@item
Functions for setting alarms and timers are discussed in @ref{Setting
an Alarm}.
@end itemize

@menu
* Processor Time::              Measures processor time used by a program.
* Calendar Time::               Manipulation of ``real'' dates and times.
* Setting an Alarm::            Sending a signal after a specified time.
* Sleeping::                    Waiting for a period of time.
* Resource Usage::              Measuring various resources used.
* Limits on Resources::         Specifying limits on resource usage.
* Priority::                    Reading or setting process run priority.
@end menu

@node Processor Time
@section Processor Time

If you're trying to optimize your program or measure its efficiency, it's
very useful to be able to know how much @dfn{processor time} or @dfn{CPU
time} it has used at any given point.  Processor time is different from
actual wall clock time because it doesn't include any time spent waiting
for I/O or when some other process is running.  Processor time is
represented by the data type @code{clock_t}, and is given as a number of
@dfn{clock ticks} relative to an arbitrary base time marking the beginning
of a single program invocation.
@cindex CPU time
@cindex processor time
@cindex clock ticks
@cindex ticks, clock
@cindex time, elapsed CPU

@menu
* Basic CPU Time::              The @code{clock} function.
* Detailed CPU Time::           The @code{times} function.
@end menu

@node Basic CPU Time
@subsection Basic CPU Time Inquiry

To get the elapsed CPU time used by a process, you can use the
@code{clock} function.  This facility is declared in the header file
@file{time.h}.
@pindex time.h

In typical usage, you call the @code{clock} function at the beginning and
end of the interval you want to time, subtract the values, and then divide
by @code{CLOCKS_PER_SEC} (the number of clock ticks per second), like this:

@smallexample
@group
#include <time.h>

clock_t start, end;
double elapsed;

start = clock();
@dots{} /* @r{Do the work.} */
end = clock();
elapsed = ((double) (end - start)) / CLOCKS_PER_SEC;
@end group
@end smallexample

Different computers and operating systems vary wildly in how they keep
track of processor time.  It's common for the internal processor clock
to have a resolution somewhere between hundredths and millionths of a
second.

In the GNU system, @code{clock_t} is equivalent to @code{long int} and
@code{CLOCKS_PER_SEC} is an integer value.  But in other systems, both
@code{clock_t} and the type of the macro @code{CLOCKS_PER_SEC} can be
either integer or floating-point types.  Casting processor time values
to @code{double}, as in the example above, makes sure that operations
such as arithmetic and printing work properly and consistently no matter
what the underlying representation is.

@comment time.h
@comment ANSI
@deftypevr Macro int CLOCKS_PER_SEC
The value of this macro is the number of clock ticks per second measured
by the @code{clock} function.
@end deftypevr

@comment time.h
@comment POSIX.1
@deftypevr Macro int CLK_TCK
This is an obsolete name for @code{CLOCKS_PER_SEC}.  
@end deftypevr

@comment time.h
@comment ANSI
@deftp {Data Type} clock_t
This is the type of the value returned by the @code{clock} function.
Values of type @code{clock_t} are in units of clock ticks.
@end deftp

@comment time.h
@comment ANSI
@deftypefun clock_t clock (void)
This function returns the elapsed processor time.  The base time is
arbitrary but doesn't change within a single process.  If the processor
time is not available or cannot be represented, @code{clock} returns the
value @code{(clock_t)(-1)}.
@end deftypefun


@node Detailed CPU Time
@subsection Detailed Elapsed CPU Time Inquiry

The @code{times} function returns more detailed information about
elapsed processor time in a @w{@code{struct tms}} object.  You should
include the header file @file{sys/times.h} to use this facility.
@pindex sys/times.h

@comment sys/times.h
@comment POSIX.1
@deftp {Data Type} {struct tms}
The @code{tms} structure is used to return information about process
times.  It contains at least the following members:

@table @code
@item clock_t tms_utime
This is the CPU time used in executing the instructions of the calling
process.

@item clock_t tms_stime
This is the CPU time used by the system on behalf of the calling process.

@item clock_t tms_cutime
This is the sum of the @code{tms_utime} values and the @code{tms_cutime}
values of all terminated child processes of the calling process, whose
status has been reported to the parent process by @code{wait} or
@code{waitpid}; see @ref{Process Completion}.  In other words, it
represents the total CPU time used in executing the instructions of all
the terminated child processes of the calling process, excluding child
processes which have not yet been reported by @code{wait} or
@code{waitpid}.

@item clock_t tms_cstime
This is similar to @code{tms_cutime}, but represents the total CPU time
used by the system on behalf of all the terminated child processes of the
calling process.
@end table

All of the times are given in clock ticks.  These are absolute values; in a
newly created process, they are all zero.  @xref{Creating a Process}.
@end deftp

@comment sys/times.h
@comment POSIX.1
@deftypefun clock_t times (struct tms *@var{buffer})
The @code{times} function stores the processor time information for
the calling process in @var{buffer}.

The return value is the same as the value of @code{clock()}: the elapsed
real time relative to an arbitrary base.  The base is a constant within a
particular process, and typically represents the time since system
start-up.  A value of @code{(clock_t)(-1)} is returned to indicate failure.
@end deftypefun

@strong{Portability Note:} The @code{clock} function described in
@ref{Basic CPU Time}, is specified by the ANSI C standard.  The
@code{times} function is a feature of POSIX.1.  In the GNU system, the
value returned by the @code{clock} function is equivalent to the sum of
the @code{tms_utime} and @code{tms_stime} fields returned by
@code{times}.

@node Calendar Time
@section Calendar Time

This section describes facilities for keeping track of dates and times
according to the Gregorian calendar.
@cindex Gregorian calendar
@cindex time, calendar
@cindex date and time

There are three representations for date and time information:

@itemize @bullet
@item 
@dfn{Calendar time} (the @code{time_t} data type) is a compact 
representation, typically giving the number of seconds elapsed since
some implementation-specific base time.
@cindex calendar time

@item
There is also a @dfn{high-resolution time} representation (the @code{struct
timeval} data type) that includes fractions of a second.  Use this time
representation instead of ordinary calendar time when you need greater
precision.
@cindex high-resolution time

@item
@dfn{Local time} or @dfn{broken-down time} (the @code{struct
tm} data type) represents the date and time as a set of components
specifying the year, month, and so on, for a specific time zone.
This time representation is usually used in conjunction with formatting
date and time values.
@cindex local time
@cindex broken-down time
@end itemize

@menu
* Simple Calendar Time::        Facilities for manipulating calendar time.
* High-Resolution Calendar::    A time representation with greater precision.
* Broken-down Time::            Facilities for manipulating local time.
* Formatting Date and Time::    Converting times to strings.
* TZ Variable::                 How users specify the time zone.
* Time Zone Functions::         Functions to examine or specify the time zone. 
* Time Functions Example::      An example program showing use of some of
                                 the time functions.
@end menu

@node Simple Calendar Time
@subsection Simple Calendar Time

This section describes the @code{time_t} data type for representing
calendar time, and the functions which operate on calendar time objects.
These facilities are declared in the header file @file{time.h}.
@pindex time.h

@cindex epoch
@comment time.h
@comment ANSI
@deftp {Data Type} time_t
This is the data type used to represent calendar time.
When interpreted as an absolute time
value, it represents the number of seconds elapsed since 00:00:00 on
January 1, 1970, Coordinated Universal Time.  (This date is sometimes
referred to as the @dfn{epoch}.)  POSIX requires that this count
ignore leap seconds, but on some hosts this count includes leap seconds
if you set @code{TZ} to certain values (@pxref{TZ Variable}).

In the GNU C library, @code{time_t} is equivalent to @code{long int}.
In other systems, @code{time_t} might be either an integer or
floating-point type.
@end deftp

@comment time.h
@comment ANSI
@deftypefun double difftime (time_t @var{time1}, time_t @var{time0})
The @code{difftime} function returns the number of seconds elapsed
between time @var{time1} and time @var{time0}, as a value of type
@code{double}.  The difference ignores leap seconds unless leap
second support is enabled.

In the GNU system, you can simply subtract @code{time_t} values.  But on
other systems, the @code{time_t} data type might use some other encoding
where subtraction doesn't work directly.
@end deftypefun

@comment time.h
@comment ANSI
@deftypefun time_t time (time_t *@var{result})
The @code{time} function returns the current time as a value of type
@code{time_t}.  If the argument @var{result} is not a null pointer, the
time value is also stored in @code{*@var{result}}.  If the calendar 
time is not available, the value @w{@code{(time_t)(-1)}} is returned.
@end deftypefun


@node High-Resolution Calendar
@subsection High-Resolution Calendar

The @code{time_t} data type used to represent calendar times has a 
resolution of only one second.  Some applications need more precision.

So, the GNU C library also contains functions which are capable of
representing calendar times to a higher resolution than one second.  The
functions and the associated data types described in this section are
declared in @file{sys/time.h}.
@pindex sys/time.h

@comment sys/time.h
@comment BSD
@deftp {Data Type} {struct timeval}
The @code{struct timeval} structure represents a calendar time.  It
has the following members:

@table @code
@item long int tv_sec
This represents the number of seconds since the epoch.  It is equivalent
to a normal @code{time_t} value.

@item long int tv_usec
This is the fractional second value, represented as the number of
microseconds.

Some times struct timeval values are used for time intervals.  Then the
@code{tv_sec} member is the number of seconds in the interval, and
@code{tv_usec} is the number of additional microseconds.
@end table
@end deftp

@comment sys/time.h
@comment BSD
@deftp {Data Type} {struct timezone}
The @code{struct timezone} structure is used to hold minimal information
about the local time zone.  It has the following members:

@table @code
@item int tz_minuteswest
This is the number of minutes west of GMT.

@item int tz_dsttime
If nonzero, daylight savings time applies during some part of the year.
@end table

The @code{struct timezone} type is obsolete and should never be used.
Instead, use the facilities described in @ref{Time Zone Functions}.
@end deftp

It is often necessary to subtract two values of type @w{@code{struct
timeval}}.  Here is the best way to do this.  It works even on some
peculiar operating systems where the @code{tv_sec} member has an
unsigned type.

@smallexample
/* @r{Subtract the `struct timeval' values X and Y,}
   @r{storing the result in RESULT.}
   @r{Return 1 if the difference is negative, otherwise 0.}  */

int
timeval_subtract (result, x, y)
     struct timeval *result, *x, *y;
@{
  /* @r{Perform the carry for the later subtraction by updating @var{y}.} */
  if (x->tv_usec < y->tv_usec) @{
    int nsec = (y->tv_usec - x->tv_usec) / 1000000 + 1;
    y->tv_usec -= 1000000 * nsec;
    y->tv_sec += nsec;
  @}
  if (x->tv_usec - y->tv_usec > 1000000) @{
    int nsec = (y->tv_usec - x->tv_usec) / 1000000;
    y->tv_usec += 1000000 * nsec;
    y->tv_sec -= nsec;
  @}

  /* @r{Compute the time remaining to wait.}
     @r{@code{tv_usec} is certainly positive.} */
  result->tv_sec = x->tv_sec - y->tv_sec;
  result->tv_usec = x->tv_usec - y->tv_usec;

  /* @r{Return 1 if result is negative.} */
  return x->tv_sec < y->tv_sec;
@}
@end smallexample

@comment sys/time.h
@comment BSD
@deftypefun int gettimeofday (struct timeval *@var{tp}, struct timezone *@var{tzp})
The @code{gettimeofday} function returns the current date and time in the
@code{struct timeval} structure indicated by @var{tp}.  Information about the
time zone is returned in the structure pointed at @var{tzp}.  If the @var{tzp}
argument is a null pointer, time zone information is ignored.

The return value is @code{0} on success and @code{-1} on failure.  The
following @code{errno} error condition is defined for this function:

@table @code
@item ENOSYS
The operating system does not support getting time zone information, and
@var{tzp} is not a null pointer.  The GNU operating system does not
support using @w{@code{struct timezone}} to represent time zone
information; that is an obsolete feature of 4.3 BSD.
Instead, use the facilities described in @ref{Time Zone Functions}.
@end table
@end deftypefun

@comment sys/time.h
@comment BSD
@deftypefun int settimeofday (const struct timeval *@var{tp}, const struct timezone *@var{tzp})
The @code{settimeofday} function sets the current date and time
according to the arguments.  As for @code{gettimeofday}, time zone
information is ignored if @var{tzp} is a null pointer.

You must be a privileged user in order to use @code{settimeofday}.

The return value is @code{0} on success and @code{-1} on failure.  The
following @code{errno} error conditions are defined for this function:

@table @code
@item EPERM
This process cannot set the time because it is not privileged.

@item ENOSYS
The operating system does not support setting time zone information, and
@var{tzp} is not a null pointer.
@end table
@end deftypefun

@comment sys/time.h
@comment BSD
@deftypefun int adjtime (const struct timeval *@var{delta}, struct timeval *@var{olddelta})
This function speeds up or slows down the system clock in order to make
gradual adjustments in the current time.  This ensures that the time
reported by the system clock is always monotonically increasing, which
might not happen if you simply set the current time.

The @var{delta} argument specifies a relative adjustment to be made to
the current time.  If negative, the system clock is slowed down for a
while until it has lost this much time.  If positive, the system clock
is speeded up for a while.

If the @var{olddelta} argument is not a null pointer, the @code{adjtime}
function returns information about any previous time adjustment that
has not yet completed.

This function is typically used to synchronize the clocks of computers
in a local network.  You must be a privileged user to use it.
The return value is @code{0} on success and @code{-1} on failure.  The
following @code{errno} error condition is defined for this function:

@table @code
@item EPERM
You do not have privilege to set the time.
@end table
@end deftypefun

@strong{Portability Note:}  The @code{gettimeofday}, @code{settimeofday},
and @code{adjtime} functions are derived from BSD.  


@node Broken-down Time
@subsection Broken-down Time
@cindex broken-down time
@cindex calendar time and broken-down time

Calendar time is represented as a number of seconds.  This is convenient
for calculation, but has no resemblance to the way people normally
represent dates and times.  By contrast, @dfn{broken-down time} is a binary
representation separated into year, month, day, and so on.  Broken down
time values are not useful for calculations, but they are useful for
printing human readable time.

A broken-down time value is always relative to a choice of local time
zone, and it also indicates which time zone was used.

The symbols in this section are declared in the header file @file{time.h}.

@comment time.h
@comment ANSI
@deftp {Data Type} {struct tm}
This is the data type used to represent a broken-down time.  The structure
contains at least the following members, which can appear in any order:

@table @code
@item int tm_sec
This is the number of seconds after the minute, normally in the range
@code{0} to @code{59}.  (The actual upper limit is @code{60}, to allow
for leap seconds if leap second support is available.)
@cindex leap second

@item int tm_min
This is the number of minutes after the hour, in the range @code{0} to
@code{59}.

@item int tm_hour
This is the number of hours past midnight, in the range @code{0} to
@code{23}.

@item int tm_mday
This is the day of the month, in the range @code{1} to @code{31}.

@item int tm_mon
This is the number of months since January, in the range @code{0} to
@code{11}.

@item int tm_year
This is the number of years since @code{1900}.

@item int tm_wday
This is the number of days since Sunday, in the range @code{0} to @code{6}.

@item int tm_yday
This is the number of days since January 1, in the range @code{0} to
@code{365}.

@item int tm_isdst
@cindex Daylight Saving Time
@cindex summer time
This is a flag that indicates whether Daylight Saving Time is (or was, or
will be) in effect at the time described.  The value is positive if
Daylight Saving Time is in effect, zero if it is not, and negative if the
information is not available.

@item long int tm_gmtoff
This field describes the time zone that was used to compute this
broken-down time value; it is the amount you must add to the local time
in that zone to get GMT, in units of seconds.  The value is like that of
the variable @code{timezone} (@pxref{Time Zone Functions}).  You can
also think of this as the ``number of seconds west'' of GMT.  The
@code{tm_gmtoff} field is a GNU library extension.

@item const char *tm_zone
This field is the name for the time zone that was used to
compute this broken-down time value.  It is a GNU library extension.
@end table
@end deftp

@comment time.h
@comment ANSI
@deftypefun {struct tm *} localtime (const time_t *@var{time})
The @code{localtime} function converts the calendar time pointed to by
@var{time} to broken-down time representation, expressed relative to the
user's specified time zone.

The return value is a pointer to a static broken-down time structure, which
might be overwritten by subsequent calls to @code{ctime}, @code{gmtime},
or @code{localtime}.  (But no other library function overwrites the contents
of this object.)

Calling @code{localtime} has one other effect: it sets the variable
@code{tzname} with information about the current time zone.  @xref{Time
Zone Functions}.
@end deftypefun

@comment time.h
@comment ANSI
@deftypefun {struct tm *} gmtime (const time_t *@var{time})
This function is similar to @code{localtime}, except that the broken-down
time is expressed as Coordinated Universal Time (UTC)---that is, as
Greenwich Mean Time (GMT) rather than relative to the local time zone.

Recall that calendar times are @emph{always} expressed in coordinated
universal time.
@end deftypefun

@comment time.h
@comment ANSI
@deftypefun time_t mktime (struct tm *@var{brokentime})
The @code{mktime} function is used to convert a broken-down time structure
to a calendar time representation.  It also ``normalizes'' the contents of
the broken-down time structure, by filling in the day of week and day of
year based on the other date and time components.

The @code{mktime} function ignores the specified contents of the
@code{tm_wday} and @code{tm_yday} members of the broken-down time
structure.  It uses the values of the other components to compute the
calendar time; it's permissible for these components to have
unnormalized values outside of their normal ranges.  The last thing that
@code{mktime} does is adjust the components of the @var{brokentime}
structure (including the @code{tm_wday} and @code{tm_yday}).

If the specified broken-down time cannot be represented as a calendar time,
@code{mktime} returns a value of @code{(time_t)(-1)} and does not modify
the contents of @var{brokentime}.

Calling @code{mktime} also sets the variable @code{tzname} with
information about the current time zone.  @xref{Time Zone Functions}.
@end deftypefun

@node Formatting Date and Time
@subsection Formatting Date and Time

The functions described in this section format time values as strings.
These functions are declared in the header file @file{time.h}.
@pindex time.h

@comment time.h
@comment ANSI
@deftypefun {char *} asctime (const struct tm *@var{brokentime})
The @code{asctime} function converts the broken-down time value that
@var{brokentime} points to into a string in a standard format:

@smallexample
"Tue May 21 13:46:22 1991\n"
@end smallexample

The abbreviations for the days of week are: @samp{Sun}, @samp{Mon},
@samp{Tue}, @samp{Wed}, @samp{Thu}, @samp{Fri}, and @samp{Sat}.

The abbreviations for the months are: @samp{Jan}, @samp{Feb},
@samp{Mar}, @samp{Apr}, @samp{May}, @samp{Jun}, @samp{Jul}, @samp{Aug},
@samp{Sep}, @samp{Oct}, @samp{Nov}, and @samp{Dec}.

The return value points to a statically allocated string, which might be
overwritten by subsequent calls to @code{asctime} or @code{ctime}.
(But no other library function overwrites the contents of this
string.)
@end deftypefun

@comment time.h
@comment ANSI
@deftypefun {char *} ctime (const time_t *@var{time})
The @code{ctime} function is similar to @code{asctime}, except that the
time value is specified as a @code{time_t} calendar time value rather
than in broken-down local time format.  It is equivalent to

@smallexample
asctime (localtime (@var{time}))
@end smallexample

@code{ctime} sets the variable @code{tzname}, because @code{localtime}
does so.  @xref{Time Zone Functions}.
@end deftypefun

@comment time.h
@comment ANSI
@deftypefun size_t strftime (char *@var{s}, size_t @var{size}, const char *@var{template}, const struct tm *@var{brokentime})
This function is similar to the @code{sprintf} function (@pxref{Formatted
Input}), but the conversion specifications that can appear in the format
template @var{template} are specialized for printing components of the date
and time @var{brokentime} according to the locale currently specified for
time conversion (@pxref{Locales}).

Ordinary characters appearing in the @var{template} are copied to the
output string @var{s}; this can include multibyte character sequences.
Conversion specifiers are introduced by a @samp{%} character, and are
replaced in the output string as follows:

@table @code
@item %a
The abbreviated weekday name according to the current locale.

@item %A
The full weekday name according to the current locale.

@item %b
The abbreviated month name according to the current locale.

@item %B
The full month name according to the current locale.

@item %c
The preferred date and time representation for the current locale.

@item %d
The day of the month as a decimal number (range @code{01} to @code{31}).

@item %H
The hour as a decimal number, using a 24-hour clock (range @code{00} to
@code{23}).

@item %I
The hour as a decimal number, using a 12-hour clock (range @code{01} to
@code{12}).

@item %j
The day of the year as a decimal number (range @code{001} to @code{366}).

@item %m
The month as a decimal number (range @code{01} to @code{12}).

@item %M
The minute as a decimal number.

@item %p
Either @samp{am} or @samp{pm}, according to the given time value; or the
corresponding strings for the current locale.

@item %S
The second as a decimal number.

@item %U
The week number of the current year as a decimal number, starting with
the first Sunday as the first day of the first week.

@item %W
The week number of the current year as a decimal number, starting with
the first Monday as the first day of the first week.

@item %w
The day of the week as a decimal number, Sunday being @code{0}.

@item %x
The preferred date representation for the current locale, but without the
time.

@item %X
The preferred time representation for the current locale, but with no date.

@item %y
The year as a decimal number, but without a century (range @code{00} to
@code{99}).

@item %Y
The year as a decimal number, including the century.

@item %Z
The time zone or name or abbreviation (empty if the time zone can't be
determined).

@item %%
A literal @samp{%} character.
@end table

The @var{size} parameter can be used to specify the maximum number of
characters to be stored in the array @var{s}, including the terminating
null character.  If the formatted time requires more than @var{size}
characters, the excess characters are discarded.  The return value from
@code{strftime} is the number of characters placed in the array @var{s},
not including the terminating null character.  If the value equals
@var{size}, it means that the array @var{s} was too small; you should
repeat the call, providing a bigger array.

If @var{s} is a null pointer, @code{strftime} does not actually write
anything, but instead returns the number of characters it would have written.

For an example of @code{strftime}, see @ref{Time Functions Example}.
@end deftypefun

@node TZ Variable
@subsection Specifying the Time Zone with @code{TZ}

In POSIX systems, a user can specify the time zone by means of the
@code{TZ} environment variable.  For information about how to set
environment variables, see @ref{Environment Variables}.  The functions
for accessing the time zone are declared in @file{time.h}.
@pindex time.h
@cindex time zone

You should not normally need to set @code{TZ}.  If the system is
configured properly, the default timezone will be correct.  You might
set @code{TZ} if you are using a computer over the network from a
different timezone, and would like times reported to you in the timezone
that local for you, rather than what is local for the computer.

In POSIX.1 systems the value of the @code{TZ} variable can be of one of
three formats.  With the GNU C library, the most common format is the
last one, which can specify a selection from a large database of time
zone information for many regions of the world.  The first two formats
are used to describe the time zone information directly, which is both
more cumbersome and less precise.  But the POSIX.1 standard only
specifies the details of the first two formats, so it is good to be
familiar with them in case you come across a POSIX.1 system that doesn't
support a time zone information database.

The first format is used when there is no Daylight Saving Time (or
summer time) in the local time zone:

@smallexample
@r{@var{std} @var{offset}}
@end smallexample

The @var{std} string specifies the name of the time zone.  It must be
three or more characters long and must not contain a leading colon or
embedded digits, commas, or plus or minus signs.  There is no space
character separating the time zone name from the @var{offset}, so these
restrictions are necessary to parse the specification correctly.

The @var{offset} specifies the time value one must add to the local time
to get a Coordinated Universal Time value.  It has syntax like
[@code{+}|@code{-}]@var{hh}[@code{:}@var{mm}[@code{:}@var{ss}]].  This
is positive if the local time zone is west of the Prime Meridian and
negative if it is east.  The hour must be between @code{0} and
@code{24}, and the minute and seconds between @code{0} and @code{59}.

For example, here is how we would specify Eastern Standard Time, but
without any daylight savings time alternative:

@smallexample
EST+5
@end smallexample

The second format is used when there is Daylight Saving Time:

@smallexample
@r{@var{std} @var{offset} @var{dst} [@var{offset}]@code{,}@var{start}[@code{/}@var{time}]@code{,}@var{end}[@code{/}@var{time}]}
@end smallexample

The initial @var{std} and @var{offset} specify the standard time zone, as
described above.  The @var{dst} string and @var{offset} specify the name
and offset for the corresponding daylight savings time time zone; if the
@var{offset} is omitted, it defaults to one hour ahead of standard time.

The remainder of the specification describes when daylight savings time is
in effect.  The @var{start} field is when daylight savings time goes into
effect and the @var{end} field is when the change is made back to standard
time.  The following formats are recognized for these fields:

@table @code
@item J@var{n}
This specifies the Julian day, with @var{n} between @code{1} and @code{365}.
February 29 is never counted, even in leap years.

@item @var{n}
This specifies the Julian day, with @var{n} between @code{0} and @code{365}.
February 29 is counted in leap years.

@item M@var{m}.@var{w}.@var{d}
This specifies day @var{d} of week @var{w} of month @var{m}.  The day
@var{d} must be between @code{0} (Sunday) and @code{6}.  The week
@var{w} must be between @code{1} and @code{5}; week @code{1} is the
first week in which day @var{d} occurs, and week @code{5} specifies the
@emph{last} @var{d} day in the month.  The month @var{m} should be
between @code{1} and @code{12}.
@end table

The @var{time} fields specify when, in the local time currently in
effect, the change to the other time occurs.  If omitted, the default is
@code{02:00:00}.

For example, here is how one would specify the Eastern time zone in the
United States, including the appropriate daylight saving time and its dates
of applicability.  The normal offset from GMT is 5 hours; since this is
west of the prime meridian, the sign is positive.  Summer time begins on
the first Sunday in April at 2:00am, and ends on the last Sunday in October
at 2:00am.

@smallexample
EST+5EDT,M4.1.0/M10.5.0
@end smallexample

The schedule of daylight savings time in any particular jurisdiction has
changed over the years.  To be strictly correct, the conversion of dates
and times in the past should be based on the schedule that was in effect
then.  However, this format has no facilities to let you specify how the
schedule has changed from year to year.  The most you can do is specify
one particular schedule---usually the present day schedule---and this is
used to convert any date, no matter when.  For precise time zone
specifications, it is best to use the time zone information database
(see below).

The third format looks like this:

@smallexample
:@var{characters}
@end smallexample

Each operating system interprets this format differently; in the GNU C
library, @var{characters} is the name of a file which describes the time
zone.

@pindex /etc/localtime
@pindex localtime
If the @code{TZ} environment variable does not have a value, the
operation chooses a time zone by default.  In the GNU C library, the
default time zone is like the specification @samp{TZ=:/etc/localtime}
(or @samp{TZ=:/usr/local/etc/localtime}, depending on how GNU C library
was configured; @pxref{Installation}).  Other C libraries use their own
rule for choosing the default time zone, so there is little we can say
about them.

@cindex time zone database
@pindex /share/lib/zoneinfo
@pindex zoneinfo
If @var{characters} begins with a slash, it is an absolute file name;
otherwise the library looks for the file
@w{@file{/share/lib/zoneinfo/@var{characters}}}.  The @file{zoneinfo}
directory contains data files describing local time zones in many
different parts of the world.  The names represent major cities, with
subdirectories for geographical areas; for example,
@file{America/New_York}, @file{Europe/London}, @file{Asia/Hong_Kong}.
These data files are installed by the system administrator, who also
sets @file{/etc/localtime} to point to the data file for the local time
zone.  The GNU C library comes with a large database of time zone
information for most regions of the world, which is maintained by a
community of volunteers and put in the public domain.

@node Time Zone Functions
@subsection Functions and Variables for Time Zones

@comment time.h
@comment POSIX.1
@deftypevar char * tzname [2]
The array @code{tzname} contains two strings, which are the standard
names of the pair of time zones (standard and daylight
savings) that the user has selected.  @code{tzname[0]} is the name of
the standard time zone (for example, @code{"EST"}), and @code{tzname[1]}
is the name for the time zone when daylight savings time is in use (for
example, @code{"EDT"}).  These correspond to the @var{std} and @var{dst}
strings (respectively) from the @code{TZ} environment variable.

The @code{tzname} array is initialized from the @code{TZ} environment
variable whenever @code{tzset}, @code{ctime}, @code{strftime},
@code{mktime}, or @code{localtime} is called.
@end deftypevar

@comment time.h
@comment POSIX.1
@deftypefun void tzset (void)
The @code{tzset} function initializes the @code{tzname} variable from
the value of the @code{TZ} environment variable.  It is not usually
necessary for your program to call this function, because it is called
automatically when you use the other time conversion functions that
depend on the time zone.
@end deftypefun

The following variables are defined for compatibility with System V
Unix.  These variables are set by calling @code{tzset}.

@comment time.h
@comment SVID
@deftypevar {long int} timezone
This contains the difference between GMT and local standard time, in
seconds.  For example, in the U.S. Eastern time zone, the value is
@code{5*60*60}.
@end deftypevar

@comment time.h
@comment SVID
@deftypevar int daylight
This variable has a nonzero value if daylight savings time rules apply.
A nonzero value does not necessarily mean that daylight savings time is
now in effect; it means only that daylight savings time is sometimes in
effect.
@end deftypevar

@node Time Functions Example
@subsection Time Functions Example

Here is an example program showing the use of some of the local time and
calendar time functions.

@smallexample
@include strftim.c.texi
@end smallexample

It produces output like this:

@smallexample
Wed Jul 31 13:02:36 1991
Today is Wednesday, July 31.
The time is 01:02 PM.
@end smallexample


@node Setting an Alarm
@section Setting an Alarm

The @code{alarm} and @code{setitimer} functions provide a mechanism for a
process to interrupt itself at some future time.  They do this by setting a
timer; when the timer expires, the process receives a signal.

@cindex setting an alarm
@cindex interval timer, setting
@cindex alarms, setting
@cindex timers, setting
Each process has three independent interval timers available:

@itemize @bullet
@item 
A real-time timer that counts clock time.  This timer sends a
@code{SIGALRM} signal to the process when it expires.
@cindex real-time timer
@cindex timer, real-time

@item 
A virtual timer that counts CPU time used by the process.  This timer
sends a @code{SIGVTALRM} signal to the process when it expires.
@cindex virtual timer
@cindex timer, virtual

@item 
A profiling timer that counts both CPU time used by the process, and CPU
time spent in system calls on behalf of the process.  This timer sends a
@code{SIGPROF} signal to the process when it expires.
@cindex profiling timer
@cindex timer, profiling

This timer is useful for profiling in interpreters.  The interval timer
mechanism does not have the fine granularity necessary for profiling
native code.
@c @xref{profil} !!!
@end itemize

You can only have one timer of each kind set at any given time.  If you
set a timer that has not yet expired, that timer is simply reset to the
new value.

You should establish a handler for the appropriate alarm signal using
@code{signal} or @code{sigaction} before issuing a call to @code{setitimer}
or @code{alarm}.  Otherwise, an unusual chain of events could cause the
timer to expire before your program establishes the handler, and in that
case it would be terminated, since that is the default action for the alarm
signals.  @xref{Signal Handling}.

The @code{setitimer} function is the primary means for setting an alarm.
This facility is declared in the header file @file{sys/time.h}.  The
@code{alarm} function, declared in @file{unistd.h}, provides a somewhat
simpler interface for setting the real-time timer.
@pindex unistd.h
@pindex sys/time.h

@comment sys/time.h
@comment BSD
@deftp {Data Type} {struct itimerval}
This structure is used to specify when a timer should expire.  It contains
the following members:
@table @code
@item struct timeval it_interval
This is the interval between successive timer interrupts.  If zero, the
alarm will only be sent once.

@item struct timeval it_value
This is the interval to the first timer interrupt.  If zero, the alarm is
disabled.
@end table

The @code{struct timeval} data type is described in @ref{High-Resolution
Calendar}.
@end deftp

@comment sys/time.h
@comment BSD
@deftypefun int setitimer (int @var{which}, struct itimerval *@var{new}, struct itimerval *@var{old})
The @code{setitimer} function sets the timer specified by @var{which} 
according to @var{new}.  The @var{which} argument can have a value of
@code{ITIMER_REAL}, @code{ITIMER_VIRTUAL}, or @code{ITIMER_PROF}.

If @var{old} is not a null pointer, @code{setitimer} returns information
about any previous unexpired timer of the same kind in the structure it
points to.

The return value is @code{0} on success and @code{-1} on failure.  The
following @code{errno} error conditions are defined for this function:

@table @code
@item EINVAL
The timer interval was too large.
@end table
@end deftypefun

@comment sys/time.h
@comment BSD
@deftypefun int getitimer (int @var{which}, struct itimerval *@var{old})
The @code{getitimer} function stores information about the timer specified
by @var{which} in the structure pointed at by @var{old}.

The return value and error conditions are the same as for @code{setitimer}.
@end deftypefun

@comment sys/time.h
@comment BSD
@table @code
@item ITIMER_REAL
@findex ITIMER_REAL
This constant can be used as the @var{which} argument to the
@code{setitimer} and @code{getitimer} functions to specify the real-time
timer.

@comment sys/time.h
@comment BSD
@item ITIMER_VIRTUAL
@findex ITIMER_VIRTUAL
This constant can be used as the @var{which} argument to the
@code{setitimer} and @code{getitimer} functions to specify the virtual
timer.

@comment sys/time.h
@comment BSD
@item ITIMER_PROF
@findex ITIMER_PROF
This constant can be used as the @var{which} argument to the
@code{setitimer} and @code{getitimer} functions to specify the profiling
timer.
@end table

@comment unistd.h
@comment POSIX.1
@deftypefun {unsigned int} alarm (unsigned int @var{seconds})
The @code{alarm} function sets the real-time timer to expire in
@var{seconds} seconds.  If you want to cancel any existing alarm, you
can do this by calling @code{alarm} with a @var{seconds} argument of
zero.

The return value indicates how many seconds remain before the previous
alarm would have been sent.  If there is no previous alarm, @code{alarm}
returns zero.
@end deftypefun

The @code{alarm} function could be defined in terms of @code{setitimer}
like this:

@smallexample
unsigned int
alarm (unsigned int seconds)
@{
  struct itimerval old, new;
  new.it_interval.tv_usec = 0;
  new.it_interval.tv_sec = 0;
  new.it_value.tv_usec = 0;
  new.it_value.tv_sec = (long int) seconds;
  if (setitimer (ITIMER_REAL, &new, &old) < 0)
    return 0;
  else
    return old.it_value.tv_sec;
@}
@end smallexample

There is an example showing the use of the @code{alarm} function in
@ref{Handler Returns}.

If you simply want your process to wait for a given number of seconds,
you should use the @code{sleep} function.  @xref{Sleeping}.

You shouldn't count on the signal arriving precisely when the timer
expires.  In a multiprocessing environment there is typically some
amount of delay involved.

@strong{Portability Note:} The @code{setitimer} and @code{getitimer}
functions are derived from BSD Unix, while the @code{alarm} function is
specified by the POSIX.1 standard.  @code{setitimer} is more powerful than
@code{alarm}, but @code{alarm} is more widely used.

@node Sleeping
@section Sleeping

The function @code{sleep} gives a simple way to make the program wait
for short periods of time.  If your program doesn't use signals (except
to terminate), then you can expect @code{sleep} to wait reliably for
the specified amount of time.  Otherwise, @code{sleep} can return sooner
if a signal arrives; if you want to wait for a given period regardless
of signals, use @code{select} (@pxref{Waiting for I/O}) and don't
specify any descriptors to wait for.
@c !!! select can get EINTR; using SA_RESTART makes sleep win too.

@comment unistd.h
@comment POSIX.1
@deftypefun {unsigned int} sleep (unsigned int @var{seconds})
The @code{sleep} function waits for @var{seconds} or until a signal
is delivered, whichever happens first.  

If @code{sleep} function returns because the requested time has
elapsed, it returns a value of zero.  If it returns because of delivery
of a signal, its return value is the remaining time in the sleep period.

The @code{sleep} function is declared in @file{unistd.h}.
@end deftypefun

Resist the temptation to implement a sleep for a fixed amount of time by
using the return value of @code{sleep}, when nonzero, to call
@code{sleep} again.  This will work with a certain amount of accuracy as
long as signals arrive infrequently.  But each signal can cause the
eventual wakeup time to be off by an additional second or so.  Suppose a
few signals happen to arrive in rapid succession by bad luck---there is
no limit on how much this could shorten or lengthen the wait.

Instead, compute the time at which the program should stop waiting, and
keep trying to wait until that time.  This won't be off by more than a
second.  With just a little more work, you can use @code{select} and
make the waiting period quite accurate.  (Of course, heavy system load
can cause unavoidable additional delays---unless the machine is 
dedicated to one application, there is no way you can avoid this.)

On some systems, @code{sleep} can do strange things if your program uses
@code{SIGALRM} explicitly.  Even if @code{SIGALRM} signals are being
ignored or blocked when @code{sleep} is called, @code{sleep} might
return prematurely on delivery of a @code{SIGALRM} signal.  If you have
established a handler for @code{SIGALRM} signals and a @code{SIGALRM}
signal is delivered while the process is sleeping, the action taken
might be just to cause @code{sleep} to return instead of invoking your
handler.  And, if @code{sleep} is interrupted by delivery of a signal
whose handler requests an alarm or alters the handling of @code{SIGALRM},
this handler and @code{sleep} will interfere.

On the GNU system, it is safe to use @code{sleep} and @code{SIGALRM} in
the same program, because @code{sleep} does not work by means of
@code{SIGALRM}.

@node Resource Usage
@section Resource Usage

@pindex sys/resource.h
The function @code{getrusage} and the data type @code{struct rusage}
are used for examining the usage figures of a process.  They are declared
in @file{sys/resource.h}.

@comment sys/resource.h
@comment BSD
@deftypefun int getrusage (int @var{processes}, struct rusage *@var{rusage})
This function reports the usage totals for processes specified by
@var{processes}, storing the information in @code{*@var{rusage}}.

In most systems, @var{processes} has only two valid values:

@table @code
@comment sys/resource.h
@comment BSD
@item RUSAGE_SELF
Just the current process.

@comment sys/resource.h
@comment BSD
@item RUSAGE_CHILDREN
All child processes (direct and indirect) that have terminated already.
@end table

In the GNU system, you can also inquire about a particular child process
by specifying its process ID.

The return value of @code{getrusage} is zero for success, and @code{-1}
for failure.

@table @code
@item EINVAL
The argument @var{processes} is not valid.
@end table
@end deftypefun

One way of getting usage figures for a particular child process is with
the function @code{wait4}, which returns totals for a child when it
terminates.  @xref{BSD Wait Functions}.

@comment sys/resource.h
@comment BSD
@deftp {Data Type} {struct rusage}
This data type records a collection usage amounts for various sorts of
resources.  It has the following members, and possibly others:

@table @code
@item struct timeval ru_utime
Time spent executing user instructions.

@item struct timeval ru_stime
Time spent in operating system code on behalf of @var{processes}.

@item long int ru_maxrss
The maximum resident set size used, in kilobytes.  That is, the maximum
number of kilobytes that @var{processes} used in real memory simultaneously.

@item long int ru_ixrss
An integral value expressed in kilobytes times ticks of execution, which
indicates the amount of memory used by text that was shared with other
processes.

@item long int ru_idrss
An integral value expressed the same way, which is the amount of
unshared memory used in data.

@item long int ru_isrss
An integral value expressed the same way, which is the amount of
unshared memory used in stack space.

@item long int ru_minflt
The number of page faults which were serviced without requiring any I/O.

@item long int ru_majflt
The number of page faults which were serviced by doing I/O.

@item long int ru_nswap
The number of times @var{processes} was swapped entirely out of main memory.

@item long int ru_inblock
The number of times the file system had to read from the disk on behalf
of @var{processes}.

@item long int ru_oublock
The number of times the file system had to write to the disk on behalf
of @var{processes}.

@item long int ru_msgsnd
Number of IPC messages sent.

@item long ru_msgrcv
Number of IPC messages received.

@item long int ru_nsignals
Number of signals received.

@item long int ru_nvcsw
The number of times @var{processes} voluntarily invoked a context switch
(usually to wait for some service).

@item long int ru_nivcsw
The number of times an involuntary context switch took place (because
the time slice expired, or another process of higher priority became
runnable).
@end table
@end deftp

An additional historical function for examining usage figures,
@code{vtimes}, is supported but not documented here.  It is declared in
@file{sys/vtimes.h}.

@node Limits on Resources
@section Limiting Resource Usage
@cindex resource limits
@cindex limits on resource usage
@cindex usage limits

You can specify limits for the resource usage of a process.  When the
process tries to exceed a limit, it may get a signal, or the system call
by which it tried to do so may fail, depending on the limit.  Each
process initially inherits its limit values from its parent, but it can
subsequently change them.

@pindex sys/resource.h
The symbols in this section are defined in @file{sys/resource.h}.

@comment sys/resource.h
@comment BSD
@deftypefun int getrlimit (int @var{resource}, struct rlimit *@var{rlp})
Read the current value and the maximum value of resource @var{resource}
and store them in @code{*@var{rlp}}.

The return value is @code{0} on success and @code{-1} on failure.  The
only possible @code{errno} error condition is @code{EFAULT}.
@end deftypefun

@comment sys/resource.h
@comment BSD
@deftypefun int setrlimit (int @var{resource}, struct rlimit *@var{rlp})
Store the current value and the maximum value of resource @var{resource}
in @code{*@var{rlp}}.

The return value is @code{0} on success and @code{-1} on failure.  The
following @code{errno} error condition is possible:

@table @code
@item EPERM
You tried to change the maximum permissible limit value,
but you don't have privileges to do so.
@end table
@end deftypefun

@comment sys/resource.h
@comment BSD
@deftp {Data Type} {struct rlimit}
This structure is used with @code{getrlimit} to receive limit values,
and with @code{setrlimit} to specify limit values.  It has two fields:

@table @code
@item rlim_cur
The current value of the limit in question.
This is also called the ``soft limit''.
@cindex soft limit

@item rlim_max
The maximum permissible value of the limit in question.  You cannot set
the current value of the limit to a larger number than this maximum.
Only the super user can change the maximum permissible value.
This is also called the ``hard limit''.
@cindex hard limit
@end table

In @code{getrlimit}, the structure is an output; it receives the current
values.  In @code{setrlimit}, it specifies the new values.
@end deftp

Here is a list of resources that you can specify a limit for.
Those that are sizes are measured in bytes.

@table @code
@comment sys/resource.h
@comment BSD
@item RLIMIT_CPU
@vindex RLIMIT_CPU
The maximum amount of cpu time the process can use.  If it runs for
longer than this, it gets a signal: @code{SIGXCPU}.  The value is
measured in seconds.  @xref{Operation Error Signals}.

@comment sys/resource.h
@comment BSD
@item RLIMIT_FSIZE
@vindex RLIMIT_FSIZE
The maximum size of file the process can create.  Trying to write a
larger file causes a signal: @code{SIGXFSZ}.  @xref{Operation Error
Signals}.

@comment sys/resource.h
@comment BSD
@item RLIMIT_DATA
@vindex RLIMIT_DATA
The maximum size of data memory for the process.  If the process tries
to allocate data memory beyond this amount, the allocation function
fails.

@comment sys/resource.h
@comment BSD
@item RLIMIT_STACK
@vindex RLIMIT_STACK
The maximum stack size for the process.  If the process tries to extend
its stack past this size, it gets a @code{SIGSEGV} signal.
@xref{Program Error Signals}.

@comment sys/resource.h
@comment BSD
@item RLIMIT_CORE
@vindex RLIMIT_CORE
The maximum size core file that this process can create.  If the process
terminates and would dump a core file larger than this maximum size,
then no core file is created.  So setting this limit to zero prevents
core files from ever being created.

@comment sys/resource.h
@comment BSD
@item RLIMIT_RSS
@vindex RLIMIT_RSS
The maximum amount of physical memory that this process should get.
This parameter is a guide for the system's scheduler and memory
allocator; the system may give the process more memory when there is a
surplus.

@comment sys/resource.h
@comment BSD
@item RLIMIT_MEMLOCK
The maximum amount of memory that can be locked into physical memory (so
it will never be paged out).

@comment sys/resource.h
@comment BSD
@item RLIMIT_NPROC
The maximum number of processes that can be created with the same user ID.
If you have reached the limit for your user ID, @code{fork} will fail
with @code{EAGAIN}.  @xref{Creating a Process}.

@comment sys/resource.h
@comment BSD
@item RLIMIT_NOFILE
@vindex RLIMIT_NOFILE
@itemx RLIMIT_OFILE
@vindex RLIMIT_OFILE
The maximum number of files that the process can open.  If it tries to
open more files than this, it gets error code @code{EMFILE}.
@xref{Error Codes}.  Not all systems support this limit; GNU does, and
4.4 BSD does.

@comment sys/resource.h
@comment BSD
@item RLIM_NLIMITS
@vindex RLIM_NLIMITS
The number of different resource limits.  Any valid @var{resource}
operand must be less than @code{RLIM_NLIMITS}.
@end table

@comment sys/resource.h
@comment BSD
@defvr Constant int RLIM_INFINITY
This constant stands for a value of ``infinity'' when supplied as
the limit value in @code{setrlimit}.
@end defvr

@c ??? Someone want to finish these?
Two historical functions for setting resource limits, @code{ulimit} and
@code{vlimit}, are not documented here.  The latter is declared in
@file{sys/vlimit.h} and comes from BSD.

@node Priority
@section Process Priority
@cindex process priority
@cindex priority of a process

@pindex sys/resource.h
When several processes try to run, their respective priorities determine
what share of the CPU each process gets.  This section describes how you
can read and set the priority of a process.  All these functions and
macros are declared in @file{sys/resource.h}.

The range of valid priority values depends on the operating system, but
typically it runs from @code{-20} to @code{20}.  A lower priority value
means the process runs more often.  These constants describe the range of
priority values:

@table @code
@comment sys/resource.h
@comment BSD
@item PRIO_MIN
@vindex PRIO_MIN
The smallest valid priority value.

@comment sys/resource.h
@comment BSD
@item PRIO_MAX
@vindex PRIO_MAX
The smallest valid priority value.
@end table

@comment sys/resource.h
@comment BSD
@deftypefun int getpriority (int @var{class}, int @var{id})
Read the priority of a class of processes; @var{class} and @var{id}
specify which ones (see below).  If the processes specified do not all
have the same priority, this returns the smallest value that any of them
has.

The return value is the priority value on success, and @code{-1} on
failure.  The following @code{errno} error condition are possible for
this function:

@table @code
@item ESRCH
The combination of @var{class} and @var{id} does not match any existing
process.

@item EINVAL
The value of @var{class} is not valid.
@end table

When the return value is @code{-1}, it could indicate failure, or it
could be the priority value.  The only way to make certain is to set
@code{errno = 0} before calling @code{getpriority}, then use @code{errno
!= 0} afterward as the criterion for failure.
@end deftypefun

@comment sys/resource.h
@comment BSD
@deftypefun int setpriority (int @var{class}, int @var{id}, int @var{priority})
Set the priority of a class of processes to @var{priority}; @var{class}
and @var{id} specify which ones (see below).

The return value is @code{0} on success and @code{-1} on failure.  The
following @code{errno} error condition are defined for this function:

@table @code
@item ESRCH
The combination of @var{class} and @var{id} does not match any existing
process.

@item EINVAL
The value of @var{class} is not valid.

@item EPERM
You tried to set the priority of some other user's process, and you
don't have privileges for that.

@item EACCES
You tried to lower the priority of a process, and you don't have
privileges for that.
@end table
@end deftypefun

The arguments @var{class} and @var{id} together specify a set of
processes you are interested in.  These are the possible values for
@var{class}:

@table @code
@comment sys/resource.h
@comment BSD
@item PRIO_PROCESS
@vindex PRIO_PROCESS
Read or set the priority of one process.  The argument @var{id} is a
process ID.

@comment sys/resource.h
@comment BSD
@item PRIO_PGRP
@vindex PRIO_PGRP
Read or set the priority of one process group.  The argument @var{id} is
a process group ID.

@comment sys/resource.h
@comment BSD
@item PRIO_USER
@vindex PRIO_USER
Read or set the priority of one user's processes.  The argument @var{id}
is a user ID.
@end table

If the argument @var{id} is 0, it stands for the current process,
current process group, or the current user, according to @var{class}.

@c ??? I don't know where we should say this comes from.
@comment Unix
@comment dunno.h
@deftypefun int nice (int @var{increment})
Increment the priority of the current process by @var{increment}.
The return value is the same as for @code{setpriority}.

Here is an equivalent definition for @code{nice}:

@smallexample
int
nice (int increment)
@{
  int old = getpriority (PRIO_PROCESS, 0);
  return setpriority (PRIO_PROCESS, 0, old + increment);
@}
@end smallexample
@end deftypefun